This invention relates generally to code division multiple access (xe2x80x9cCDMAxe2x80x9d) cellular telephone systems and, more particularly, to a method of optimally managing forward link power in a CDMA system.
A typical cellular communication system is comprised of multiple cell sites covering a geographic region. Each cell site supports radiotelephone communication with a mobile unit within a service area through at least one antenna and a base station transceiver substation (xe2x80x9cBTSxe2x80x9d). The mobile unit is sometimes called a subscriber unit, and the mobile unit supports the voice and data communication from a subscriber or mobile unit user in the cell site service area. The BTS, sometimes called the base station, provides wireless communications coverage within the cell site service area by performing base station processing to support the CDMA common air interface to the mobile units. Mobile units in the cell site area will communicate through the antenna thereby supporting the radio communication to the BTS.
Multiple BTS units for cell site areas are coupled to a base station controller (xe2x80x9cBSCxe2x80x9d) that will support coded voice communications and allow for the continuity of calls as mobile units travel through the cellular network. The BSC communicates with the mobile switching center (xe2x80x9cMSCxe2x80x9d) which routes calls handled by the BSC and BTS units associated with the MSC to the public switched telephone network (xe2x80x9cPSTNxe2x80x9d). The cellular system may have multiple MSCs, each controlling communication traffic on a number of cell site areas. Third Generation wireless networks, which have focused on Internet applications, may adopt a more distributed architecture with network interconnection directly to the BSC.
If the cell site has a limited traffic load, a single omnidirectional antenna will be assigned to cover the entire cell site area. If traffic load in a cell site area is heavier, the cell site area can be divided into a number of sectors wherein each sector has an antenna assigned to a particular section of the cell site area. For example, a cell site area divided into three sectors may have three antennas assigned to cover 120xc2x0 areas of the cell site area. In this manner, the number of communication links from the BTS to the mobile unit, called forward links, can be increased through the use of cell site sectors. The communications from the mobile unit to the base station BTS are called the reverse link.
Power Allocation on Forward Link
The forward link power is the power allocated to the communication channel from the BTS to the mobile unit. The BTS determines how much power to dedicate to each mobile unit in the cell site area so as to maintain sufficient signal quality. The BTS controls the amount of power allocated to each mobile unit on the system based on the distance of the mobile unit from the antenna assigned to the cell or sector, as well as other factors such as shadow fading (buildings, terrain obstacles) and small scale fading (relay losses, multipath fading, local scattering losses). The BTS can also adjust the overall power of the signal transmitted to the mobile unit.
The number of simultaneous users allowed to use the cellular communication system in a particular cell site area is restricted by the amount of forward link power available to support communications with the mobile units. More specifically, the maximum number of users, or capacity on the cell site, is limited by the number of forward links that can be supported at any given time and the amount of power available to the BTS to transmit the forward link signals. As such, the allocation of forward link power is one of the most important resources in the cellular communication system.
Increasing Power Allocations and Interference
In order to support more users per cell site area and increase capacity on the cellular system, it would make sense for the cell site to increase the power allocated to the forward link communications to be shared among more users. Increasing this forward link power allocation, however, has a significant negative effect on surrounding cell site areas because of signal interference created by the increased radiated power. This interference in surrounding cell site areas can be overcome by a corresponding increase in the amount of forward link signal power in these surrounding cells. The increase in signal power in the surrounding cell site areas, however, increases the total amount of interference in the system, as well as the interference level in the first cell site.
Accordingly, increasing the forward link power signal is not the solution to increasing capacity on the system. In fact, increasing the forward link power leads to a redundant cycle of the greater interference, and then, the need for greater signal power to overcome that interference. To avoid this outcome, the BTS in each cell site area have been known to maintain the signal power in a manner that is less than the ratio of forward link signal power to power creating interference, also called Pdesired signal/Pinterference. This ratio is maintained in order to maintain an acceptable level of interference while transmitting the desired signal at an acceptable power level for signal quality.
The present invention provides greater capacity in a dynamic allocation of power. First, the invention requires the reduction, not the increase, of signal power in low traffic areas. Because this signal power reduction reduces interference in surrounding high traffic cell site areas, those high traffic cell site areas can re-direct some of this surplus power to support a greater voice and data traffic load. Accordingly, the invention increases capacity of voice and data communications in high traffic areas, where it is needed most, by lowering (and dynamically regulating) the signal power in low traffic cell site areas.
In the CDMA system, there are traffic channels and overhead channels. The traffic channels carry voice and data communications, while the overhead channels supervise and control the communications traffic on the cellular system. The overhead channels include a pilot signal, paging channel, synchronization channel, and a control channel. The system must always allocate some of the available forward link power for the support of these overhead channels.
When a mobile unit moves from one cell site area to another cell site area, the continuity of the call is maintained by a xe2x80x9csoft hand-offxe2x80x9d routine. In the xe2x80x9csoft hand-off routine,xe2x80x9d the antenna assigned to handle the communication between the mobile units will be switched to the new antenna in the second cell site area. When a mobile unit moves from one cell site sector to another sector (but within the same cell site area), the continuity of the call must be maintained by a xe2x80x9csofter hand-off routine.xe2x80x9d
New users entering a new sector or cell site area via these xe2x80x9csoft hand-offxe2x80x9d and xe2x80x9csofter hand-offxe2x80x9d routines have an extremely high priority in the allocation of available power and communication channels because the continuity of existing calls in the cellular system is extremely important. The system must always allocate a portion of the available forward link power to handle mobile units entering the sector or cell site area via a hand-off routine. After the forward link power has been allocated for overhead channel support and hand-off reserves, the remaining forward link power is used to support voice and data traffic channels.
The present invention reduces signal power requirements in the system by reducing, and dynamically regulating, the power in low traffic cell site areas. The reduced signal power used in these low traffic cell site areas will reduce the interference in surrounding the cell site areas. With reduced interference in the surrounding cell site areas, the surrounding cell sites can redirect some of the power formerly used to overcome higher levels of interference. This redirected surplus power can be used to support more traffic channels thereby increasing the capacity of the surrounding cell site area. The call capacity is increased in the system without increasing the overall power level at the cell sites and with a decrease in the overall signal interference on the cellular network.